Decode the Slanted Ciphertext LeetCode solution in Java

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class Solution {
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  public String decodeCiphertext(String str, int rows) {
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    // first find column size!!
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    int cols = str.length() / rows;
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    StringBuilder res = new StringBuilder(), new_res = new StringBuilder();
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    ;
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    for (int i = 0; i < cols; i++) {
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      // iterating diagonally!!
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      for (int j = i; j < str.length(); j += cols + 1) res.append(str.charAt(j));
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    }
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    // removing last spaces!!!
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    int fg = 0;
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    for (int i = res.length() - 1; i >= 0; i--) {
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      if (fg == 0 && res.charAt(i) == ' ') continue;
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      fg = 1;
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      new_res.append(res.charAt(i));
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    }
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    return new_res.reverse().toString();
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  }
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}